Chemical passivation is a process where an inert, or passive, film is formed over the surface of a material. In lithium batteries, passivation occurs when an inert film is formed over the lithium anode. This film is the product of a chemical reaction between the lithium anode and the electrolyte that takes place when the battery not being discharged.
The passive film has a high ionic conductivity. Because the film is conductive, it is removed from the anode when the battery is discharged. However, when the battery is not being discharged, the passive film re-grows over the anode.
There are two main problems caused by the passive film. First, if the passive film is too large, the battery will not operate. Second, if the passive film is uneven, the battery will be a potential safety hazard.
First, if the battery is stored for a long time, the passive film will grow very large. If the passive film is too large, the amount of electric current needed to remove the film from the anode cannot be generated by the electrochemical potential of the cell. The large passive layer over the anode will prevent the battery from working.
Second, the initial growth of the passive film over the surface of the anode is consistent and even. However, as time passes, the growth of the film becomes uneven, due to local conditions of the anode and defects in the film. Accordingly, the conductivity of this uneven, non-homogeneous film is similarly uneven. The film's conductivity varies as the thickness of the film varies over the anode of the battery. Therefore, when the battery is discharged, the distribution of current over the anode will be uneven. The current will be concentrated at certain points of the anode. These concentrated points of current will be disconnected from each other and from other points of the anode. The current concentration and the anode disconnect can cause violent venting in lithium batteries.
There is a need for a device and a method which will remove the passive layer from the anode of a battery when the battery is inoperable.
There is a need for a device and a method which will safely remove the passive layer from the anode of a battery without causing anode disconnect or current concentration.